Photoelectric conversion element, image reading device, image forming apparatus, and photoelectric conversion method

ABSTRACT

A photoelectric conversion element includes a light receiving element, a buffer unit, a current control circuit, and an elimination circuit. The light receiving element generates electrical charge according to an amount of light received. The buffer unit buffers and outputs a voltage signal according to the electrical charge generated by the light receiving element. When the buffer unit outputs the voltage signal, the current control circuit controls electric current flowing through the buffer unit so as to be a predetermined amount of electric current. The elimination circuit eliminates high-frequency components in a band equal to or higher than a predetermined band from the voltage signal output from the buffer unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by referencethe entire contents of Japanese Patent Application No. 2014-211891 filedin Japan on Oct. 16, 2014.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoelectric conversion element, animage reading device, an image forming apparatus, and a photoelectricconversion method.

2. Description of the Related Art

As a photoelectric conversion element used in a scanner, a CCD has beenused conventionally; however, to cope with recent demand for speedimprovement, a CMOS linear image sensor (a CMOS sensor) has attractedattention. The CMOS sensor is the same as the CCD in the point thatincident light is photoelectrically converted by a photodiode (PD).However, the CMOS sensor differs from the CCD in that the CMOS sensorperforms a charge-voltage conversion near a pixel and outputs theconverted voltage to a subsequent stage. Furthermore, a CMOS process isused in the CMOS sensor, so the CMOS sensor can have a circuit such asan analog-digital converter (ADC) built-in, and therefore has anadvantage over the CCD in high-speed performance.

A CMOS linear image sensor is composed of source followers with respectto each pixel and current loads which supply a bias current to thesource followers, thereby achieving fast signal readout. However, theCMOS linear image sensor has a problem that when a current load isadded, i.e., when electrical current applied to the source followers isincreased, noise is worsened. Especially, high-frequency noise cannot beeliminated by correlated double sampling (CDS) and therefore causesfixed pattern noise (FPN), resulting in the occurrence of verticalstripes on an image.

To cope with the above-described problem, for example, Japanese PatentApplication Laid-open No. 2010-178117 has disclosed an amplificationtype solid-state imaging device that outputs and writes a signal outputfrom an amplifying transistor, which amplifies a signal output from aphotoelectric conversion unit, to a capacitance in a period where theamplifying transistor goes into a metastable state since the amplifyingtransistor has borne only a capacitance load and, after a write switchunit performed initialization of the capacitance, has moved from asaturation region operation to a subthreshold region operation.

However, the amplification type solid-state imaging device disclosed inJapanese Patent Application Laid-open No. 2010-178117 has a problem thatFPN is worsened due to a limitation on the signal response speed.

Therefore, it is desirable to provide a photoelectric conversionelement, an image reading device, an image forming apparatus, and aphotoelectric conversion method capable of reducing fixed pattern noisewhile securing the necessary response speed.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve theproblems in the conventional technology.

According to an aspect of the present invention, there is provided aphotoelectric conversion element including: a light receiving elementthat generates electrical charge according to an amount of lightreceived; a buffer unit that buffers and outputs a voltage signalaccording to the electrical charge generated by the light receivingelement; a current control circuit that controls, when the buffer unitoutputs the voltage signal, electric current flowing through the bufferunit so as to be a predetermined amount of electric current; and anelimination circuit that eliminates high-frequency components in a bandequal to or higher than a predetermined band from the voltage signaloutput from the buffer unit.

According to another aspect of the present invention, there is provideda photoelectric conversion method performed by a photoelectricconversion element, the method including: generating, by a lightreceiving element, electrical charge according to an amount of lightreceived; buffering and outputting, by a buffer unit, a voltage signalaccording to the electrical charge generated at the light receivingelement; controlling, by a current control circuit, when the buffer unitoutputs the voltage signal, electric current flowing through the bufferunit so as to be a predetermined amount of electric current; andeliminating, by an elimination circuit, high-frequency components in aband equal to or higher than a predetermined band from the voltagesignal output from the buffer unit.

According to still another aspect of the present invention, there isprovided a photoelectric conversion element including: light receivingmeans for generating electrical charge according to an amount of lightreceived; buffer means for buffering and outputting a voltage signalaccording to the electrical charge generated by the light receivingmeans; current control means for controlling, when the buffer meansoutputs the voltage signal, electric current flowing through the buffermeans so as to be a predetermined amount of electric current; andelimination means for eliminating high-frequency components in a bandequal to or higher than a predetermined band from the voltage signaloutput from the buffer means.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an entire configuration of aphotoelectric conversion element;

FIG. 2 is a diagram showing configurations of a pixel, a pixel circuit,and a storage unit that the photoelectric conversion element includes;

FIG. 3 is a diagram showing the timing at which the pixel circuit writesa signal on a memory capacitance;

FIG. 4A is a diagram showing a noise spectrum in the photoelectricconversion element;

FIG. 4B is a diagram showing a noise spectrum in the photoelectricconversion element;

FIG. 5 is a diagram showing a configuration example of a pixel unit forreduction of noise;

FIG. 6 is a diagram showing the state of a signal read out from thepixel circuit to the storage unit;

FIG. 7 is a diagram showing a noise spectrum in a photoelectricconversion element having the configuration shown in FIG. 5;

FIG. 8 is a diagram showing a configuration example of a pixel unit of aphotoelectric conversion element according to an embodiment;

FIG. 9 is a diagram showing a signal read out from the pixel circuit tothe storage unit through a band limiting unit;

FIG. 10 is a diagram showing a noise spectrum in a photoelectricconversion element including the pixel unit shown in FIG. 8;

FIG. 11 is a diagram showing a configuration of a pixel unit with fewercircuits added than the configuration shown in FIG. 2;

FIG. 12 is a diagram showing a signal read out from the pixel circuit toa storage unit shown in FIG. 11;

FIG. 13 is a diagram showing a configuration of a pixel unit in aphotoelectric conversion element including a CDS unit;

FIG. 14A is a diagram showing a reason why high-frequency noise cannotbe corrected by CDS;

FIG. 14B is a diagram showing a reason why high-frequency noise cannotbe corrected by CDS;

FIG. 15 is a diagram showing a configuration example of a pixel unithaving a function corresponding to the band limiting unit;

FIG. 16A is a diagram showing the operation of a photoelectricconversion element including the pixel unit shown in FIG. 15;

FIG. 16B is a diagram showing the operation of a photoelectricconversion element including the pixel unit shown in FIG. 15;

FIG. 17 is a diagram showing a noise spectrum in a photoelectricconversion element including a pixel unit provided with a storage unit;

FIG. 18 is a diagram illustrating a configuration of a photoelectricconversion element according to the embodiment;

FIG. 19 is a flowchart showing a band-limitation adjusting method;

FIG. 20A is a diagram showing a state before adjustment;

FIG. 20B is a diagram showing a state before adjustment;

FIG. 21A is a diagram showing a state after the adjustment;

FIG. 21B is a diagram showing a state after the adjustment; and

FIG. 22 is a diagram showing an outline of an image forming apparatusincluding an image reading device provided with, for example, thephotoelectric conversion element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the context that led up to the present invention is explained.FIG. 1 is a diagram illustrating an entire configuration of a CMOSlinear image sensor (a photoelectric conversion element) 10. A PIX(R)20, a PIX(G) 22, and a PIX(B) 24 each include about seven thousand PDs(photodiodes: light receiving elements), and are configured for red (R),green (G), and blue (B) colors, respectively. The PDs are arranged inone direction with respect to each color of light received, and generatean electrical charge according to an amount of light received.Furthermore, a PIX_BLK(R) 21, a PIX_BLK(G) 23, and a PIX_BLK(B) 25 eachinclude about seven thousand pixel circuits (PIX_BLKs), and areconfigured for RGB colors, respectively. That is, each PD is providedwith a pixel circuit (PIX_BLK).

Each pixel circuit (PIX_BLK) converts an electrical charge accumulatedby a PD into a voltage signal, and outputs the signal to an analogmemory through a read line. The PIX_BLK is composed of a transfertransistor that transfers the electrical charge of the PD to a floatingdiffusion (FD), a reset transistor that resets the FD, and a sourcefollower transistor that buffers and outputs an FD voltage to the readline. Unlike an area sensor, a linear sensor reads out a signal fromeach of RGB pixels independently, so a read line exists with respect toeach pixel independently.

An AMEM 26 includes, for example, about seven thousand analog memories(such as Cs to be described later) with respect to each of RGB colors,and holds a signal with respect to each pixel and sequentially outputsimage signals on a column-by-column basis. As this AMEM 26 holdssignals, a global shutter system in which the operation timings, i.e.,the exposure timings of PIX and PIX_BLK are the same among RGB isachieved.

An ADC 27 includes as many AD converters as the number of columns, andsequentially converts image signals from analog to digital on acolumn-by-column basis. The ADC 27 includes as many AD converters as thenumber of columns and performs parallel processing, thereby achievingthe speed improvement as a photoelectric conversion element whilesuppressing the operating speed of the AD converters.

The signals subjected to the A/D conversion by the ADC 27 are held by aparallel-serial conversion unit (P/S) 28 with respect to each pixel, andthe held signals are sequentially output to an LVDS 29. On the upstreamside of the P/S 28, the photoelectric conversion element 10 processesparallel data that each column of pixels in a main scanning directionhave been subjected to parallel processing; on the other hand, on thedownstream side of the P/S 28, the photoelectric conversion element 10processes serial data for each of RGB colors. The LVDS 29 converts asignal output from the P/S 28 into a low-voltage differential serialsignal, and outputs the converted signal to a subsequent stage. A timingcontrol unit (TG) 30 controls the units composing the photoelectricconversion element 10.

FIG. 2 is a diagram showing configurations of a pixel 200, a pixelcircuit (PIX_BLK) 210, and a storage unit 261 that the photoelectricconversion element 10 includes. The pixel 200, the pixel circuit 210,and the storage unit 261 compose a pixel unit in the photoelectricconversion element 10. The photoelectric conversion element 10 includes,for example, about seven thousand pixel units per color. Specifically,in the photoelectric conversion element 10, for example, the PIX(R) 20has about seven thousand pixels 200, the PIX_BLK(R) 21 has about seventhousand pixel circuits 210, and the AMEM 26 includes about seventhousand storage units 261. The same is true on the other colors (GB).

The pixel 200 includes a PD (a light receiving element) thatphotoelectrically converts an incident light. The PD outputs anaccumulated electrical charge to the pixel circuit 210. The pixelcircuit 210 includes a floating diffusion (FD) that performscharge-voltage conversion, a reset transistor that resets the FD, atransfer transistor that transfers an electrical charge of the PD to theFD, and a source follower (SF) that buffers and outputs a signal to asubsequent stage. The signal from the SF is read out to the subsequentstage through a read wiring. That is, the SF is a buffer unit thatbuffers and outputs a voltage signal according to an electrical chargegenerated by the PD. The pixel circuit 210 is connected to thesubsequent storage unit 261.

The storage unit 261 includes a selecting switch (SL) that selects apixel 200, a current source (Is) that supplies a bias current to the SF,a selecting switch (S) that selects the storage unit 261, and a memorycapacitance (an analog memory: Cs). The storage unit 261 outputs asignal to an above-described AD converter. The current source (Is) is acurrent control circuit that controls the electric current flowingthrough the buffer unit so as to be a predetermined amount of electriccurrent if the buffer unit outputs a voltage signal.

Incidentally, the photoelectric conversion element 10 adopts a globalshutter that the operation to write on the memory capacitance (Cs) issimultaneously performed on all RGB pixels; however, from the operationof readout from the memory capacitance (Cs), the photoelectricconversion element 10 performs serial processing of sequentially readingRGB three pixels out to a subsequent stage.

FIG. 3 is a diagram showing the timing at which the pixel circuit 210writes a signal on the memory capacitance (Cs). When the photoelectricconversion element 10 reads out a signal accumulated in a PD, an SFoutputs, i.e., a signal (Vsf) is output through a read wiring, and thepixel selecting switch (SL) and the memory-capacitance selecting switch(S) are turned ON. When a light signal is read out, the signal becomes asignal of which the signal level is reduced from an initial state (areset level) to a signal level according to the light signal.

At this time, the pixel circuit 210 can give a response about a changein the signal Vsf at high speed because the storage unit 261 includesthe current source (Is), and can write the signal on the memorycapacitance (Cs) at high speed. This is because an electric currentrequired for the charge and discharge (the discharge here) of the memorycapacitance (Cs) is sufficiently secured due to the current source (Is).

However, there is a problem that having a current source causes anincrease in noise. This is because while a fast signal response is givenby the current source, it allows to be followed by high-frequency noise.For example, when high-frequency noise has been generated just like inVsf shown in FIG. 3, Vsf varies at writing period end timing at whichwriting on the memory capacitance (Cs) ends (hold timing), and a signallevel (Vs) with an error (A) with respect to an original level(indicated by a dotted line in FIG. 3) is written on the memorycapacitance (Cs).

At this time, an error A (generation of noise) differs from pixel topixel in general, so fixed pattern noise (FPN) is generated, resultingin the occurrence of vertical stripes on an image. Incidentally, in FIG.3, high-frequency noise is illustrated as single-frequency noise for thesake of convenience; however, high-frequency noise is actually whitenoise including various frequency components. Furthermore, although notillustrated in FIG. 3, Vsf is at a reset level while a control signal RSis ON, and becomes at a signal level when RS has turned OFF and T hasturned ON.

As described above, the current source Is allows for fast signalreadout, but causes a problem of FPN due to an increase in noise,resulting in deterioration in the image quality. Furthermore, theinfluence of high-frequency noise cannot be eliminated by correlateddouble sampling (CDS), and therefore it is necessary to suppress thenoise.

FIG. 4 is a diagram showing noise spectrums in the photoelectricconversion element 10. As shown in FIG. 4A, the photoelectric conversionelement 10 has characteristics that noise exists in the entire frequencyband and the noise intensity decreases toward the high-frequency side(1/f noise).

However, as shown in FIG. 4B, in the photoelectric conversion element10, the band (frequency width) of cumulative noise (the product of aunit frequency and an amount of noise: an energy spectrum) isoverwhelmingly wide on the high-frequency side, so the noisecontribution rate increases toward the high-frequency side. Therefore,to reduce FPN or aliasing noise, it is important to suppresshigh-frequency noise.

FIG. 5 is a diagram showing a configuration example of a pixel unit forreduction of noise. The pixel unit shown in FIG. 5 differs from thepixel unit shown in FIG. 2 in that the pixel unit shown in FIG. 5includes a storage unit 262 having no current source Is.

FIG. 6 is a diagram showing the state of a signal read out from thepixel circuit 210 to the storage unit 262. When a signal accumulated ina PD is read out, a point that an SF output (Vsf) is reduced from theinitial state (the reset level) to a signal level according to a lightsignal is the same as in FIG. 3. However, as the storage unit 262 is notprovided with a current source Is, the SF operates in a state wherealmost no current flows through the SF (a subthreshold regionoperation). Therefore, a Vsf signal response is limited, thereby thespeed of signal writing on the memory capacitance (Cs) is also limited.This is because an electric current required for the charge anddischarge (the discharge here) of the memory capacitance (Cs) isinsufficient, so it takes time to respond.

Incidentally, an initial voltage of Vs shown in FIG. 3 is set at a lowlevel; however, in the configuration shown in FIG. 2, there is nodischarge path for the memory capacitance (Cs), so the Vs potential canchange only in a direction of voltage reduction.

In a case of the above-described subthreshold region operation, the Vsfresponse speed, i.e., the signal band is limited. Therefore,high-frequency noise shown in FIG. 3 is suppressed, and there is noerror in Vs due to noise. However, meanwhile, the signal response speedis limited; therefore, it takes time for Vsf to reach an intended level.This is not really matter when the operating speed of the photoelectricconversion element 10 is slow; however, when the photoelectricconversion element 10 performs high-speed operation, Vs is fixed (held)before it reaches an intended signal level. Therefore, just like thecase shown in FIG. 3, there is an error (Δ) with respect to a signallevel supposed to be written originally (indicated by a dotted line),and FPN is generated due to the limitation on a response instead ofsuppressing FPN caused by high-frequency noise.

As described above, when the photoelectric conversion element 10performs the subthreshold region operation without a current source Is,noise can be suppressed; however, due to the limitation on a signalresponse, FPN is newly generated.

FIG. 7 is a diagram showing a noise spectrum in the photoelectricconversion element 10 having the configuration shown in FIG. 5. Thenoise spectrum in FIG. 7 has the same characteristics as the exampleshown in FIG. 4A in that noise exists in the entire frequency band andthe noise intensity decreases toward the high-frequency side. However,in the configuration shown in FIG. 5, the signal responsiveness (band)is greatly limited, so noise is reduced overall.

Incidentally, a zone up to a frequency fa in FIG. 7 indicates a bandrequired for a Vsf signal response. This is equivalent to alow-frequency side signal band, and this band being limited means thatthe signal responsiveness is limited.

FIG. 8 is a diagram showing a configuration example of a pixel unit of aphotoelectric conversion element according to an embodiment. To reduceFPN, a noise band has to be limited while securing the necessary signalresponsiveness. In the pixel unit of the photoelectric conversionelement according to the embodiment, the storage unit 261 includes acurrent source (Is), and the pixel unit further includes a band limitingunit 400 that limits a noise band independently of the source followerof the pixel circuit 210. Specifically, as shown in FIG. 8, the pixelunit of the photoelectric conversion element according to the embodimentis provided with the band limiting unit (LIM) 400 in a stage subsequentto the SF of the pixel circuit 210.

The band limiting unit 400 is composed of, for example, a switch (VR),and is arranged in series between the pixel circuit 210 and the storageunit 261. In the band limiting unit 400, a filter is composed of an ONresistance and memory capacitance (Cs) of the switch (VR), thereby an SFoutput signal band can be easily limited. That is, the band limitingunit 400 is an elimination circuit that eliminates high-frequencycomponents in a band equal to or higher than a predetermined band from avoltage signal output from the buffer unit. A voltage by the switch (VR)is direct voltage, and the switch (VR) is always in an ON state when asignal is read out from the pixel circuit 210.

Here, a band according to the band limiting unit 400 and the memorycapacitance (Cs) is set so that the noise band is limited while securingthe necessary signal responsiveness (within a scope which does notaffect the signal followability). Furthermore, the ON resistance of theswitch (VR) is easily set to an arbitrary value by setting the switchsize and a control signal voltage.

FIG. 9 is a diagram showing a signal read out from the pixel circuit 210to the storage unit 261 through the band limiting unit 400. In anexample shown in FIG. 9, when a signal accumulated in a PD is read out,points that an SF output (Vsf) is reduced from the initial state (thereset level) to the signal level and high-frequency noise issuperimposed on Vsf are the same as the example shown in FIG. 3.

However, the band of the high-frequency noise superimposed on Vsf islimited by the band limiting unit 400, so high-frequency noise issuppressed in Vlim. In the configuration shown in FIG. 8, the bandlimiting unit 400 is configured independently of the source follower, sothe SF output signal band can be limited while maintaining thehigh-speed operation of the source follower; therefore, it is possibleto optimize the limitation on a band so as not to affect the signalresponsiveness (a period of writing on the memory capacitance (Cs)).Therefore, in the configuration shown in FIG. 8, a signal Vlim withnoise suppressed is held in the memory capacitance (Cs) without fallinginto the lack of a response like in FIG. 6.

As described above, the band limiting unit 400 is provided in betweenthe storage unit 261 including the current source Is and the pixelcircuit 210, thereby it is possible to limit the band so as not toaffect the period of writing on the memory capacitance (Cs); therefore,it is possible to suppress FPN caused by high-frequency noise or thelack of a response.

FIG. 10 is a diagram showing a noise spectrum in a photoelectricconversion element including the pixel unit shown in FIG. 8. The noisespectrum in FIG. 10 has the same characteristics as in FIG. 4A in thatnoise exists in the entire frequency band and the noise intensitydecreases toward the high-frequency side. However, as shown in FIG. 10,in the configuration of the pixel unit shown in FIG. 8, noise on thehigh-frequency side is reduced as compared with the characteristic shownin FIG. 4A (indicated by a dotted line in FIG. 10).

Here, noise on the low-frequency side is not reduced so as to secure thesignal responsiveness. However, as explained with FIG. 4, as circuitnoise in the photoelectric conversion element, the high-frequency sidenoise is overwhelmingly large and therefore has a small effect.Incidentally, as explained with FIG. 7, the zone up to the frequency faindicates a band required for a Vsf signal response, and the pixel unitshown in FIG. 8 optimizes the band thereby securing the Vsfresponsiveness.

FIG. 11 is a diagram showing a configuration of a pixel unit with fewercircuits added than the configuration shown in FIG. 2. In theconfiguration shown in FIG. 8, the band limiting unit 400 is providedindependently of the source follower (SF); however, as shown in FIG. 11,the band can be limited by using a switch that a storage unit 263 has.That is, the storage unit 263 is configured so that a pixel selectingswitch (SL) doubles as a band limiting function. When the pixel unitincludes the storage unit 263, the effect of limiting the band is thesame as the configuration shown in FIG. 8.

FIG. 12 is a diagram showing a signal read out from the pixel circuit210 to the storage unit 263 shown in FIG. 11. As shown in FIG. 12, whena signal is written on the memory capacitance (Cs), control signals ofthe pixel switch SL and the memory selecting switch S are set to Highlevel, thereby the pixel switch SL and the memory selecting switch S areturned ON; however, the storage unit 263 limits the band by setting theHigh-level control signal of the pixel switch SL to Low level.

The storage unit 263 uses a change in an ON resistance of a MOStransistor by a change in gate voltage. The storage unit 263 performsthe band limitation by setting the High level of the pixel switch SL tobe lower than that in the storage unit 261, thereby increasing the ONresistance. A signal input to a gate of the pixel switch SL is a controlsignal for switching the ON/OFF state of the pixel switch SL; thisON/OFF switching is performed in the same manner as in FIG. 3.Incidentally, the pixel switch SL (a MOS switch) of the storage unit 263is an NMOS, so a value of High level is changed; however, when it iscomposed of a PMOS, a value of Low level only has to be changed.Furthermore, the gate of the pixel switch SL is a node to which anarbitrary voltage can be applied in the photoelectric conversion element10; however, it can be a node to which an arbitrary voltage can beapplied by the outside through a terminal.

The High level of a control signal input to the pixel switch SL can bechanged. Accordingly, even when high-frequency noise superimposed on Vsfvaries among the PDs, the band limitation can be set appropriately. Whenthe band limitation is performed by means of the pixel switch SL (a MOSswitch) included in the storage unit 263 as described above, the limitedband is changed by changing the amplitude of a control signal, therebyFPN caused by individual differences among the PDs can be reduced.

Incidentally, the pixel selecting switch SL of the storage unit 263 hasthe function of the band limiting unit 400; alternatively, thememory-capacitance selecting switch (S) can be configured to have thefunction of the band limiting unit 400. Furthermore, in the storage unit263, the ON resistance of the MOS switch is configured to be able to bechanged so that the band can be changed; alternatively, a value ofmemory capacitance can be configured to be able to be changed.

Subsequently, a configuration of a pixel unit in a photoelectricconversion element including a CDS unit that performs correlated doublesampling is explained. FIG. 13 is a diagram showing the configuration ofthe pixel unit in the photoelectric conversion element including the CDSunit. There is a photoelectric conversion element that realizes CDS bynot only holding the signal level in a memory capacitance (Cs) of astorage unit 264 but also holding the reset level in a memorycapacitance (Cr). This is a technology to correct FPN by extracting thenet signal level only by subtracting the reset level from the signallevel of a pixel; however, the influence of high-frequency noise cannotbe eliminated by CDS.

FIG. 14 is diagram showing a reason why high-frequency noise cannot becorrected by CDS. When CDS is performed by using the pixel unit shown inFIG. 13, a memory-capacitance selecting switch R is in an ON state in astate where the pixel selecting switch SL is ON, and the reset level isfirst written on a memory capacitance. Then, the memory-capacitanceselecting switch (S) is in an ON state, and the signal level is written.

FIG. 14A shows a CDS operation when high-frequency noise issuperimposed. At the end of a reset-level writing period, a signal isheld, and Vr is determined; however, the level of Vsf at the end ofwriting is the same as an ideal level without noise (indicated by adotted line), so the ideal level of Vr is written. Then, at the end of asignal-level writing period, Vs is determined; however, the Vsf level ofVs at the end of writing is a level which deviates from an ideal leveldue to the influence of noise, so the level of Vs deviating by A fromthe ideal level is written. As a result, Vs−Vr subtracted by CDS is leftwith the error A from the ideal level, and therefore the influence ofhigh-frequency noise cannot be corrected by CDS.

On the other hand, FIG. 14B shows a CDS operation when low-frequencynoise is superimposed. The process of determining Vr and Vs is the same;however, the CDS operation differs in that an error (Δr) due to noise iswritten in Vr, and about the same error (Δs) as Δr is written in Vs.This is because in the case of low-frequency noise, the time to changethe level is long, so when a CDS sampling period (an interval betweenwriting of the reset level and writing of the signal level) is shortwith respect to the noise period, there is little difference betweensignal variation in the reset level and signal variation in the signallevel.

Therefore, Vs−Vr subtracted by CDS has a minor deviation from the ideallevel, and the influence of low-frequency noise can be corrected by CDS.Incidentally, the effect of CDS on low-frequency noise is determined bythe noise period and the CDS sampling period; therefore, if thelow-frequency noise is noise having a period about two or more timeslonger than the CDS period, the influence of the noise can beeliminated.

As described above, the influence of low-frequency noise can beeliminated by CDS; however, high-frequency noise cannot be eliminated.Incidentally, FIG. 14 illustrates an example of a dark output state (thereset level≈the signal level) for the sake of simplicity.

FIG. 15 is a diagram showing a configuration example of a pixel unithaving a function corresponding to the band limiting unit 400, thememory capacitance (Cs), and the memory capacitance (Cr). It isdescribed above that high-frequency noise cannot be eliminated by CDS;however, in other words, that means the influence of low-frequency noisecan be eliminated. Furthermore, as shown in FIG. 10, the configurationshown in FIG. 8 secures the responsiveness, and therefore cannot limitlow-frequency noise.

A storage unit 265 can minimize the band to be limited, i.e., theinfluence on responsiveness by setting the pixel selecting switch (SL)as a band limiting unit that limits the band which cannot be correctedby CDS. As described above, a photoelectric conversion element includinga pixel unit provided with the storage unit 265 enables the storage unit265 to suppress high-frequency noise and enables low-frequency noise tobe corrected by CDS, and therefore can suppress FPN in the entirefrequency band.

FIG. 16 is diagram showing the operation of a photoelectric conversionelement including the pixel unit shown in FIG. 15. FIG. 16A shows a CDSoperation when high-frequency noise is superimposed in the pixel unitshown in FIG. 15. The writing of the reset level and the signal level isthe same as in FIG. 14. However, in FIG. 16A, in Vlim, high-frequencynoise superimposed on Vsf is suppressed by the storage unit 265.Accordingly, no-error levels of Vr and Vs are written. Therefore, thereis no error in Vs−Vr subtracted by CDS.

On the other hand, FIG. 16B shows a CDS operation when low-frequencynoise is superimposed. The storage unit 265 cannot limit thelow-frequency noise, so the noise superimposed on Vsf is alsosuperimposed on Vlim. The subsequent operation is the same as in FIG.14B, and because of the low-frequency noise, a deviation in Vs−Vrsubtracted by CDS is minor and its influence can be corrected.

As described above, the photoelectric conversion element including thepixel unit provided with the storage unit 265 enables the storage unit265 to suppress high-frequency noise and enables low-frequency noise tobe corrected by CDS, and therefore can suppress FPN in the entirefrequency band.

FIG. 17 is a diagram showing a noise spectrum in the photoelectricconversion element including the pixel unit provided with the storageunit 265. The noise spectrum in FIG. 17 has the same characteristics asin FIG. 10 in that noise exists in the entire frequency band and thenoise intensity decreases toward the high-frequency side. However, inFIG. 17, noise on the high-frequency side is reduced as compared withthe example shown in FIG. 4A (indicated by a dotted line in FIG. 17).Here, the noise spectrum in FIG. 17 differs from that shown in FIG. 10in that noise on the low-frequency side is not reduced so as to securethe signal responsiveness and the noise band being able to be correctedby CDS is not limited. In this case, the band being able to be correctedby CDS is on the high-frequency side rather than the band required for aresponse; therefore, the influence on responsiveness can be minimized bynot limiting the CDS band.

Incidentally, the zone up to the frequency fa indicates a band requiredfor a Vsf signal response as described with FIG. 7, and a zone up to fbindicates a noise band being able to be corrected by CDS. Furthermore,the example shown in FIG. 17 shows a state where the band is limited inorder from the outside of the band being able to be corrected by CDS,i.e., the uncorrectable band; however, the storage unit 265 can limitthe band in order from the band being able to be corrected by CDS.

FIG. 18 is a diagram illustrating a configuration of a photoelectricconversion element 10 a according to the embodiment. Incidentally, outof components of the photoelectric conversion element 10 a shown in FIG.18, substantially the same component as the photoelectric conversionelement 10 (shown in FIG. 1) is assigned the same reference numeral. AnAMEM 26 a includes a band limiting unit train 40. The band limiting unittrain 40 includes, for example, about seven thousand band limiting units400 with respect to each color, and suppresses high-frequency noise.Furthermore, the AMEM 26 a includes the memory capacitance (Cs) and thememory capacitance (Cr). Incidentally, instead of the band limiting unittrain 40, the AMEM 26 a can be configured to include about seventhousand storage units 265 with respect to each color.

Then, signals with noise suppressed by the AMEM 26 a are read out withrespect to each pixel and held into each memory capacitance in the AMEM26 a as well, and the held signals are sequentially read out onto theADC with respect to each of RGB colors. As this AMEM 26 a holds signals,a global shutter system in which the operation timings, i.e., theexposure timings of the pixel 200 and the pixel circuit 210 are the sameamong RGB is achieved.

The ADC 27 includes as many AD converters as the number of columns, andsequentially converts image signals from analog to digital on acolumn-by-column basis. A digital CDS (DCDS) 31 performs CDS using thereset level and signal level output from the ADC 27. A timing controlunit (TG) 30 a controls the units composing the photoelectric conversionelement 10 a. The band limiting units 400 can be configured to beincluded in the pixel circuit 210.

The photoelectric conversion element 10 a can suppress vertical stripescaused by FPN. Incidentally, in a case of an area sensor, FPN isgenerated with respect to each of pixels arranged in two dimensions, soalthough S/N deterioration is generated, but it is not such fataldeterioration in the image quality as vertical stripes.

Subsequently, a method for adjusting the band limitation is explained.FIG. 19 is a flowchart showing the band-limitation adjusting method. Inthe pixel unit shown in FIG. 11, the band limitation by the bandlimiting unit 400 can be changed, thereby the pixel unit can respond toindividual differences in noise among the PDs; yet, if the band isadjusted by detecting the FPN level, the optimum band can be set withrespect to each individual.

As shown in FIG. 19, at the start of the adjustment, a user firstacquires FPN data (Step S100). The FPN data can be easily acquired byacquiring image data in a dark state. The user determines whether thelevel acquired through “FPN data acquisition” is equal to or less than athreshold (Step S102). When the level is equal to or less than thethreshold (YES at Step S102), the user ends off the adjustment; however,when the level exceeds the threshold (NO at Step S102), the user changesthe limited band through “limited-band adjustment” (Step S104).

In the process at Step S104, as shown in FIG. 11, the band is changed bychanging VR voltage, and, in this case, varies in a direction of furtherlimiting the band to lower the FPN level. Then, the user again acquiresFPN data (Step S100), and determines whether the acquired level is equalto or less than the threshold (Step S102). As described above, theoptimum band can be set by adjusting the limited band with respect toeach individual. Incidentally, the above is a basic adjustment method,and an upper limit can be set on the number of times of loop processingin the threshold determination or band adjustment, or the limited bandcan be calculated by an arithmetic operation according to a value of FPNlevel to reduce the number of times of loop processing.

Subsequently, the operation of the photoelectric conversion element inthe limited-band adjustment is explained. FIG. 20 shows a state beforethe adjustment; FIG. 20A is a diagram showing main-scanning output datain the dark state, and FIG. 20B is a diagram showing a noise spectrum.Here, a difference between the maximum and minimum values in an outputlevel distribution of the main-scanning-direction output data is definedas an FPN level.

A frequency fb in the noise spectrum is an upper limit noise frequencybeing able to be corrected by CDS, and fc indicates a cutoff frequencyof a band limited by a band limiting unit. Before the band adjustmentshown in FIG. 20, a value of fc is higher than fb when viewed from thenoise spectrum; therefore, the band being unable to be corrected by CDScannot be limited entirely. Therefore, as shown in the main-scanninglevel distribution, some degree of FPN level is generated.

FIG. 21 shows a state after the adjustment. In the band adjustment, theband to be adjusted is changed so that fc falls below fb as shown inFIG. 21 after the adjustment. Then, the noise band being unable to becorrected by CDS, which has been unable to be limited in FIG. 20, islimited, and the FPN level is reduced as shown in the main-scanninglevel distribution.

Subsequently, an image forming apparatus including an image readingdevice provided with the photoelectric conversion element 10 a accordingto the embodiment is explained. FIG. 22 is a diagram showing an outlineof an image forming apparatus 50 including an image reading device 60provided with, for example, the photoelectric conversion element 10 a.The image forming apparatus 50 is, for example, a copier ormultifunction peripheral (MFP) including the image reading device 60 andan image forming unit 70.

The image reading device 60 includes, for example, the photoelectricconversion element 10 a, an LED driver (LED DRV) 600, and an LED 602.The LED driver 600 drives the LED 602 in synchronization with a linesynchronization signal output from the timing control unit (TG) 30 a orthe like. The LED 602 irradiates an original with light. Insynchronization with a line synchronization signal or the like, thephotoelectric conversion element 10 a receives a reflected light fromthe original, and a plurality of PDs (not shown) starts generating andaccumulating electric charge. Then, after the photoelectric conversionelement 10 a performs an AD conversion, a parallel-serial conversion,etc., the photoelectric conversion element 10 a outputs image data tothe image forming unit 70.

The image forming unit 70 includes a processing unit 80 and a printerengine 82; the processing unit 80 and the printer engine 82 areconnected via an interface (I/F) 84.

The processing unit 80 includes an LVDS 800, an image processing unit802, and a CPU 804. The CPU 804 controls the units, such as thephotoelectric conversion element 10 a, composing the image formingapparatus 50. Furthermore, the CPU 804 (or the timing control unit 30)controls the PDs so that the PDs each start generating electric chargeaccording to an amount of light received at almost the same time.

The photoelectric conversion element 10 a outputs, for example, imageddata of an image read by the image reading device 60, a linesynchronization signal, a transmission clock, etc. to the LVDS 800. TheLVDS 800 converts the received imaged data, line synchronization signal,transmission clock, etc. into parallel 10-bit data. The image processingunit 802 performs image processing using the converted 10-bit data, andoutputs the imaged data etc. to the printer engine 82. The printerengine 82 prints out the received imaged data.

According to the present invention, it is possible to reduce fixedpattern noise while securing the necessary response speed.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

What is claimed is:
 1. A photoelectric conversion element comprising: alight receiving element that generates electrical charge according to anamount of light received; a buffer unit that buffers and outputs avoltage signal according to the electrical charge generated by the lightreceiving element; a current control circuit that controls, when thebuffer unit outputs the voltage signal, electric current flowing throughthe buffer unit so as to be a predetermined amount of electric current;and an elimination circuit that eliminates high-frequency components ina band equal to or higher than a predetermined band from the voltagesignal output from the buffer unit.
 2. The photoelectric conversionelement according to claim 1, wherein the elimination circuit includes aMOS transistor, and the band of high-frequency components to beeliminated is determined in advance according to size of anon-resistance of the MOS transistor.
 3. The photoelectric conversionelement according to claim 2, wherein the elimination circuit includes anode to which an arbitrary voltage can be applied so that a value of theon-resistance of the MOS transistor can be changed.
 4. The photoelectricconversion element according to claim 1, further comprising a CDS unitthat performs correlated double sampling on the voltage signal outputfrom the buffer unit, wherein the elimination circuit eliminateshigh-frequency components in a band higher than a band of frequencycomponents that the CDS unit can eliminate from the voltage signaloutput from the buffer unit.
 5. The photoelectric conversion elementaccording to claim 1, wherein the light receiving elements are arrangedin one direction with respect to each color of light received.
 6. Thephotoelectric conversion element according to claim 5, wherein theelimination circuit eliminates high-frequency components so that fixedpattern noise generated in the light receiving element is equal to orless than a predetermined threshold.
 7. An image reading deviceaccording to claim 1, comprising the photoelectric conversion element ofclaim
 1. 8. An image forming apparatus according to claim 7, comprising:the image reading device of claim 7; and an image forming unit thatforms an image on the basis of output of the image reading device.
 9. Aphotoelectric conversion method performed by a photoelectric conversionelement, the method comprising: generating, by a light receivingelement, electrical charge according to an amount of light received;buffering and outputting, by a buffer unit, a voltage signal accordingto the electrical charge generated at the light receiving element;controlling, by a current control circuit, when the buffer unit outputsthe voltage signal, electric current flowing through the buffer unit soas to be a predetermined amount of electric current; and eliminating, byan elimination circuit, high-frequency components in a band equal to orhigher than a predetermined band from the voltage signal output from thebuffer unit.
 10. A photoelectric conversion element comprising: lightreceiving means for generating electrical charge according to an amountof light received; buffer means for buffering and outputting a voltagesignal according to the electrical charge generated by the lightreceiving means; current control means for controlling, when the buffermeans outputs the voltage signal, electric current flowing through thebuffer means so as to be a predetermined amount of electric current; andelimination means for eliminating high-frequency components in a bandequal to or higher than a predetermined band from the voltage signaloutput from the buffer means.